PROJECT SUMMARY Fanconi anemia (FA/BRCA) pathway is comprised of at least 17 proteins that maintain genomic stability and prevent cancer. Bi-allelic germline disruption of any FA gene causes Fanconi anemia (FA), a genetic disorder causing bone marrow failure and high risk of cancer. Somatic mutations of FA/BRCA genes occur in spontaneous cancers. Thus, disruption of FA/BRCA signaling promotes malignancies in both inherited syndromes and the general population. The FA tumor suppressor network controls multiple genome-housekeeping checkpoints. In addition to the well-established roles of FA proteins in interphase DNA replication/repair, the FA pathway controls mitosis, including the spindle assembly checkpoint (SAC), a tumor suppressor network that regulates chromosome segregation. The SAC is regulated by several tumor suppressors, including MAD2, and SAC impairment predisposes to aneuploidy and cancer. However, the mechanisms causing abnormal SAC function upon loss of FA remain largely unknown. Furthermore, the in vivo clinical significance of SAC dysfunction in the pathogenesis of FA-associated cancers needs to be explored before the development of future therapeutic strategies targeting the weakened SAC in FA-deficient cells can become a reality. We hypothesize that SAC dysfunction contributes to the in vivo development of aneuploidy and cancer upon loss of FA signaling. To test this hypothesis, we have generated a novel Fancc-/-; Mad2+/- mouse model, in which Mad2 heterozygosity further weakens SAC function in the FA-deficient background. In Aim 1, we will determine whether these Fancc-/-; Mad2+/- mice are cancer-prone. Utilizing cells from these animals, we will employ a micronucleus test to assess the relative contribution of SAC dysfunction to the development of aneuploidy in comparison to other known factors such as interphase DNA damage repair. Additionally, we aim to dissect the mechanism by which loss of FANCC contributes to erroneous chromosome segregation and impaired mitosis by defining the relationship between FANCC and its binding partner CDK1, a well-known regulator of the metaphase-to-anaphase transition (Aim 2). This proposal will lead to a better understanding of SAC dysfunction as a driver of genomic instability and tumorigenesis in the context of FA signaling inactivation, and ultimately contribute to the development of mitotic-centered therapies for FA-associated tumors.